The regulation of gene activation will be studied in preimplantation mouse embryos using the structural gene (Gus) of the well characterized enzyme Beta-glucuronidase. The timing of expression of this enzyme in early embryos was established using a microfluorometric assay capable of measuring the activities of single cells. The role of various factors in controlling the timing of transcription and translation of the Gus gene will be examined after various experimental manipulations of the embryos using this assay in conjunction with mutants of the Gus gene. Specifically, I will examine 1) whether the timing of expression is a property of the whole embryo or of single cells, 2) whether the timing of expression is influenced by the numbers of cell cleavages, cell size or chronological time from fertilization, and 3) whether there are mutants with altered timing of expression.